1. Field of the Invention
This invention relates to an evaporative emission control system for an internal combustion engine, and more particularly to an evaporative emission control system of this kind which is adapted to control emission of evaporative fuel, including HC, which is adsorbed by a charcoal canister, into the intake system of the engine.
2. Prior Art
Evaporative emission control systems have conventionally been widely used, which control emission of evaporative fuel generated in a fuel tank of an automotive vehicle during stoppage of an internal combustion engine installed therein, into the atmosphere. In such systems, evaporative fuel is adsorbed by a charcoal canister during stoppage of the engine, and is emitted (purged) from the canister during operation of the engine, as disclosed e.g. by Japanese Utility Model Publication (Kokoku) No. 60-21494.
An evaporative fuel emission control system disclosed in this publication is adapted to control an amount of evaporative fuel to be emitted (purged) depending on load on the engine by the use of a restriction hole and a flow rate control valve arranged in parallel with each other in an evaporative fuel-purging passage for purging evaporative fuel adsorbed by a canister into the intake system of an internal combustion engine, and a two-state valve arranged in series with the flow rate control valve. The flow rate control valve starts to open when load on the engine reaches a first predetermined value, and thereafter increases its opening as the load on the engine increases, while the two-state valve closes when the load on the engine is below a second predetermined value lower than the first predetermined value, and also when it is above a third predetermined value higher than the first predetermined value. In this manner, the amount of evaporative fuel purged is restricted under a low load condition of the engine to secure traveling stability of the vehicle under such a low engine load condition, increased under a medium load condition of the engine to stabilize the traveling of the vehicle, and stopped under a high engine load condition when a high engine output is required, to thereby further stabilize the traveling of the vehicle.
However, in the evaporative emission control system constructed as above, negative pressure or intake vacuum is supplied from a portion of the intake pipe upstream of a throttle valve, for controlling the operation of the two-state valve, so that when the engine is idling with the throttle valve closed, pressure within the portion of the intake pipe upstream of the throttle valve becomes equal to atmospheric pressure to close the two-state valve, which makes it impossible to perform purging of evaporative fuel from the canister into the intake system.
On the other hand, if the engine continues to be idling for a long time, evaporative fuel generated in a fuel tank of the engine is supplied to the canister and finally hydrocarbons HC start to be emitted from the canister through an air inlet port thereof. Therefore, particularly in very hot areas of the world, it is required to perform purging of evaporative fuel into the intake system even while the engine is idling.
A possible solution to this problem, i.e. means for performing purging of evaporative fuel even while the engine is idling, would be to provide a duty control valve in the evaporative fuel-purging passage, which is adapted to have its valve opening ratio (duty ratio) controlled according to an amount of intake air, and arrange such that negative pressure is supplied from a portion of the intake pipe downstream of the throttle valve for controlling the operation of the duty control valve. When the engine is idling, however, load on the engine is low, i.e. the amount of intake air is small, and hence it is necessary to control the amount of purged evaporative fuel within a small value range. However, it is difficult for the duty control valve to perform a small flow rate control by duty ratios smaller than 25%. Therefore, there can occur variations in the flow rate of evaporative fuel purged so that the flow rate is unstable, which not only results in degraded exhaust emission characteristics due to fluctuations in the air-fuel ratio of a mixture supplied to the engine but also brings about a problem of offensive noises repeatedly produced by opening and closing motions of the duty control valve.
On the other hand, there is a high possibility that evaporative fuel having a high concentration is supplied to the engine when purging of evaporative fuel is carried out immediately after the start of the engine, because a three way catalyst provided in the exhaust system has not been fully activated due to a low temperature of the engine immediately after the start of the engine. This also causes a problem of unburnt hydrocarbons HC adversely affecting the exhaust emission characteristics.